1. Field of the Invention
The present invention is directed to an indirect fired process heater for heating process fluids such as natural gas or oil or any liquids or any gas. In particular, the present invention is directed to an indirect fired process heater wherein heat transfer fluid is heated in order to heat the process fluid.
2. Prior Art
Indirect fired process heaters are known to heat process fluids such as a liquid or a gas which might be employed in chemical, petroleum, or other industrial applications. For example, natural gas in a pipe that passes through a pipeline transmission/distribution system may be periodically heated for transmission purposes. Keeping the natural gas above a certain temperature will prevent water from condensing and/or freezing in or on a natural gas pipeline. Another industrial application would be as a preheater for further processing, such as natural gas processing. A further application of indirect fired process heaters is in fuel gas conditioning units.
In a standard indirect fired process heater, a quantity of heat transfer fluids is initially heated in a vessel with the fluids remaining static in the vessel. Heat retained by the heat transfer fluid is transferred to the process fluid. Thus, the process fluid is indirectly heated rather than directly heated. An indirect fired process heater provides more uniform temperature control than a direct fired heater and also reduces the likelihood of fire or explosion when heating combustible process fluids such as natural gas. The heat transfer fluid may be of different types, one type being a mixture of glycol and water. Ethylene glycol, propylene glycol or other types of glycol might be utilized.
Sams (U.S. Pat. No. 5,921,206) discloses an example of a conventional indirect process fluid heater with a novel baffle system. As in indirect fired process heaters to date, the entire vessel would be filled with heat transfer fluid medium.
It would be desirable to provide an indirect fired process heater which is more efficient than existing indirect fired process heaters.
It would be desirable to provide an indirect fired process heater that requires less heat process fluid to be heated than conventionally required for an equivalent output.
It would also be desirable to provide an indirect fired process heater that can start up from cold shutdown condition to full flow operation in a substantially shorter time period than a conventional indirect fired heater.
It would be desirable to provide an indirect fired process heater wherein the length of the heater could be decreased and the weight of the heater could be decreased from a conventional indirect fired heater.
It would be desirable to provide an indirect fired heater that can operate with low-nox burners which will reduce nox.
The present invention provides an improved indirect fired process heater apparatus and method. The apparatus includes a toroidal shell having an outer cylinder and a smaller diameter inner cylinder. The outer cylinder and inner cylinder together form a fluid tight enclosure for containing heat transfer fluid.
A plurality of helical heat transfer fluid coils are positioned within the toroidal shell and are coaxial therewith. The helical heat transfer coils have a radius less than the inner cylinder. The heat transfer fluid coils contain a heat transfer fluid which passes therein and therethrough. The heat transfer fluid is directed from the heat transfer fluid coils through a line into the toroidal shell where the heat transfer fluid circulates and thereafter is returned by a pump via a line back to the heat transfer fluid coils. A closed loop, circulating system is thereby formed.
A burner at one end of the vessel supplies heat to an axial passageway formed by the helical heat transfer fluid coils. Heat from the burner is directed into and through the axial passageway by a fan, fan/blower or natural draft type burners. The heat directed by the fan/blower or natural gas burner passes generally axially through the axial passageway.
A plurality of helical process fluid heating coils are positioned within the apparatus and are coaxial with but independent from the heat transfer fluid coils. The process fluid heating coils pass through the toroidal shell so that the process coils are in heat exchange relationship with the heat transfer fluid. The helical process fluid coils each have an axial diameter which is intermediate between the outer cylinder and the inner cylinder. The process fluid, such as natural gas, enters through an intake, passes through the helical process fluid heating coils, and thereafter exits through an outlet.
Hot combustion products (hereinafter referred to as xe2x80x9cflue gasesxe2x80x9d) generated by the burner passes into and through the axial passageway and thereafter reverses direction and passes through an annulus formed by the exterior of the heat transfer fluid coils and the inner cylinder of the toroidal shell. Thereafter, these cooled flue gases are permitted to move out of an exhaust stack extending radially from the apparatus.